Fluoropolymers, such as poly(tetrafluoroethylene) (PTFE), poly(vinylidine fluoride) (PVDF), poly(vinylidine-co-hexafluoropropene) (VDF/HFP elastomer), etc., exhibit a unique combination of properties, including thermal stability, chemical inertness (acid and oxidation resistance), low water and solvent absorptivities, self-extinguishing, excellent weatherability, and very interesting surface properties. They are commonly used in many high-end applications, such as aerospace, automotive, textile finishing, and microelectronics. However, fluoropolymers also have some drawbacks, including limited processibility, poor adhesion to substrates, limited crosslinking chemistry, and inertness to chemical modification, which limit their applications when interactive and reactive properties are paramount. In the past decades, many research groups have focused on the preparation of functional fluoropolymers containing particular functional groups. In general, there are two approaches to achieving the direct incorporation of functional groups into fluoropolymers during the polymerization process, including (i) controlling the polymerization using functional initiators or chain transfer agents to prepare telechelic fluoropolymers containing terminal functional groups, and (ii) copolymerizing of fluorinated monomers with functional comonomers to form functional fluoro-copolymers containing pendent functional groups.
The first method of using functional initiator was pioneered by Rice and Sandberg at the 3M Company (see U.S. Pat. No. 3,461,155). They reported the preparation of low molecular weight telechelic VDF/HFP elastomers containing two ester terminal groups by using a diester peroxide initiator. The average functionality of the resulting telechelic VDF/HFP elastomer was not reported. However, it is logical to expect some difficulties in achieving a perfect telechelic structure with the functionality of two in each polymer, which requires all the propagating radicals involving radical coupling reaction in the termination step. Saint-Loup et al. (see Macromolecules, 35, 1542, 2002) also attempted to prepare telechelic VDF/HFP elastomers containing two opposing hydroxy terminal groups by using hydrogen peroxide as an initiator. Several advantages of using hydrogen peroxide initiator include cost, high reactivity, and directly forming hydroxy terminal groups. However, many side reactions are also occur in this polymerization, and the final product contains not only hydroxy terminal groups but also carboxylic acid terminal groups, as well as some unsaturated terminal groups. To improve the hydroxy content of hydroxy fluoroelastomers, the polymerization method requires an additional step, which is the reduction of any carboxylic acid group into opposing hydroxy end groups using a strong reducing agent, such as LiAlH4, which sometimes generates more unsaturated double bond and causes the decoloration of the product.
In the late 1970's and early 1980's, Oka et al. disclosed an interesting iodine transfer polymerization (ITP) method to prepare telechelic fluoropolymers containing two terminal iodine groups. (see Comtemp. Topics Polym. Sci., 4, 763, 1984; U.S. Pat. No. 4,158,678) The chemistry is based on the combination of a reversible addition-fragmentation chain transfer (RAFT) process and an α,ω-diiodoperfluoroalkane (I—RF—I) chain transfer agent, where RF is CF2CF2, CF2CF2CF2CF2, CF2CFCl, CF2CF(CF3), etc. The living characteristics are usually demonstrated by the increase of molecular weight with conversion of monomer and relatively narrow molecular weight distribution (Mw/Mn<2). The active CF2—I groups are always located at both ends of the polymer chain and maintain similar reactivity despite the growing polymer molecular weight. This reaction process has led to an important commercial product, i.e. a diiodo-terminated VDF/HFP elastomer with the trade name Dai-E1®, which is a liquid rubber at room temperature and is readily curable via heating or radiation to form a 3-D network that has excellent heat oil, solvent, chemical and ozone resistance, and high mechanical strength and low compression set. It is useful as a sealing material for O-ring, gaskets, tubes, valves and bellows, as well as useful in linings, protective gloves, and shoes. In addition, this iodo-terminated telechelic also provides new route to preparing segmental polymers (block or graft copolymers), which is composed of two or more different polymer segments.
Theoretically, the most effective way to prepare functional fluoropolymers is by copolymerization of the fluorinated monomers with functional comonomers. Numerous attempts have met with limited success. Several new functional monomers have been synthesized and studied, including non-fluorinated comonomers (having a CH2═CH- vinyl group) and fluorinated comonomers (having a CF2═CF- vinyl group) (J. of Fluorine Chem., 104, 53, 2000).
In general, random copolymerization between the fluorinated monomer and non-fluorinated monomer is very difficult, due to the opposite e values (the inductive effect of the monomer). In fact, several reported copolymerization reactions between fluorinated monomers and non-fluorinated monomers, such as ethylene, vinyl ether, and N-vinyl pyrroridone, showed a strong tendency of forming alternative copolymer structures (see Boutevin, et al. Macromol. Symp. 82, 1, 1994). In addition, the introduction of the non-fluorinated functional comonomers significantly degrade the properties of the final products. To preserve the desirable fluoropolymer properties, the fluorinated functional comonomers containing a CF2═CF- vinyl group that is incorporated into the perfluorinated polymer backbone after polymerization is preferably used during the copolymerization reaction.
In general, the perfluorinated functional comonomers are expensive chemicals. Among these comonomers, perfluorovinyl functional monomers, i.e. CF2═CF(CF2)n(CH2)mX, where n=0–10, m=1–4, and X=—OH, —COOH or epoxy group, developed by Daikin Company are most interesting in copolymerization reactions (U.S. Pat. No. 4,544,720). They are very effective in the copolymerization reactions to achieve good incorporation and high copolymer molecular weight. Recently, Ameduri, et al. also reported several trifluorovinyl functional monomers, i.e. CF2═CF(CH2)mX, where m=1–3 and X=—OH, OCOCH3, —COOH, SO3H, epoxy, thiol functional groups, which were synthesized via telomerization reaction. (see J. Applied Polym. Sci. 73, 189, 1999, J. Fluorine Chem., 93, 117, 1999; J. Fluorine Chem., 114, 171, 2002). However, these functional comonomers usually give low yields, less than 20%, and low molecular weight copolymers, less than 3,000 g/mole. This is due to the existence of allylic hydrogen atoms in the comonomers. As is well-known in the art, regular free radical polymerization of monomers having allylic hydrogen facilitates chain transfer reaction, therefore reducing the polymer molecular weight and catalyst activity.
Additionally, the preparation of fluoropolymers typically requires special reaction conditions, partially due to the relatively low reactivity that is attributed from electron deficiency and to the lack of resonance in the double bond. In the past decades, two processes have evolved for the polymerization of fluorinated monomer—suspension and emulsion processes in aqueous solution. Many catalyst systems have been employed including the inorganic peroxides such as potassium, sodium, or ammonium persulfate, and organic peroxides such as dibenzoyl peroxide, diacetyl peroxide or di-tert-butylperoxide. Both inorganic and organic initiators have some disadvantages. The inorganic peroxy initiators produce polymers with less processability and somewhat thermal instability while organic peroxide initiators require extreme conditions in polymerization, such as high pressure, high temperature, and long reaction time to achieve a reasonable yield.
In the past few years, Chung et al. have developed new radical initiators that are relatively stable and can initiate living radical polymerization at ambient temperature. The chemistry was based on the mono-oxidation adducts of trialkylborane as the living radical initiator. The original research objective was centered around the functionalization of polyolefins by first incorporating borane groups into a polymer chain, which was then selectively oxidized by oxygen to form the mono-oxidized borane moieties that initiated free radical graft-form polymerization at ambient temperature to form polyolefin graft and block copolymers (Chung, et. al, U.S. Pat. Nos. 5,286,800 and 5,401,805; Macromolecules, 26, 3467, 1993; Macromolecules, 31, 5943, 1998; J. Am. Chem. Soc., 121, 6763, 1999). Several relatively stable radical initiators were discovered, which exhibited living radical polymerization characteristics, with a linear relationship between polymer molecular weight and monomer conversion and producing block copolymers by sequential monomer addition (see Chung, et.al, U.S. Pat. Nos. 6,420,502 and 6,515,088, J. Am. Chem. Soc., 118, 705, 1996). This stable radical initiator system was recently extended to the polymerization of fluorinated monomers, which can effectively occur in bulk and solution conditions. Some interesting ferroelectric fluoro-terpolymers, showing large electromechanical response, have been prepared with high molecular weight and controlled polymer structure with narrow molecular weight and composition distributions (see Chung, et al., U.S. Pat. No. 6,355,749; Macromolecules, 35, 7678, 2002).
However, there is a continuing need for convenient methods for the synthesis of fluoropolymers having one or more functional groups.